The present invention is concerned with a multi-junction solar cell that employs an optical system to provide extremely high solar flux, resulting in a system that produces very efficient electrical output. More particularly, the invention is directed to a solar energy system that combines a non-imaging light concentrator, or flux booster, with a Kohler homogenizer primary and secondary mirror subsystem, wherein the non-imaging concentrator is efficiently coupled to the mirrors such that uniform irradiance is achieved for high intensity light concentration onto the multi-junction solar cell.
Solar cells for electrical energy production are very well known but have limited utility due to the very high Kwh cost of production. While substantial research has been ongoing for many years, the cost per Kwh still is about ten times that of conventional electric power production. In order to even compete with wind power or other alternative energy sources, the efficiency of production of electricity from solar cells must be drastically improved.
Related prior art is described in (Winston, Gordon, Optics Letters, 2005), which considers a two mirror aplanatic system (which produce sharp imaging of normal-incidence rays on the cell center while satisfying the Abe sine condition) and which may be combined with a non-imaging concentrator. FIG. 1 shows such a two-mirror aplanatic system without the non-imaging concentrator. Primary mirror 10 concentrates the light onto the secondary mirror 11, which illuminates the solar cell 12. This system has a clear limitation in that the illumination on the solar cell it can achieve is highly non-uniform, which reduces the cell efficiency and system reliability. This is because the optics is imaging the plane at infinity onto the plane of the target, where the cell is placed, and thus the sun is imaged on the cell. The angular acceptance of this two-mirror aplanatic concentrator is several times (for example 3) greater than the angular size of the sun to allow for tolerances. The imaging mapping makes the acceptance angle to sun angle ratio the same as the cell diameter to sun image diameter ratio. Therefore, the area of the round target would be 32 times greater than that of the solar image. If the average concentration of the prior art design is 500 suns, the local concentration can reach as much as 32×500=4,500 suns. This concentration value cannot be tolerated by present high-efficiency multifunction cells, which show an abrupt drop in efficiency if they operate above 2,000-3,000 suns.
Other related prior art is disclosed in a paper by L. W. James, “Use of imaging refractive secondaries in photovoltaic concentrators”, SAND89-7029, Alburquerque, N. Mex., 1989. In that paper, a Kohler integrator system is used as a photovoltaic concentrator (FIG. 2). The Kohler integrator consists of two imaging optical elements (primary and secondary) with positive focal length (that is, producing a real image of an object at infinity). The secondary is placed at the focal plane of the primary, and the secondary images the primary onto the cell. In James' paper, the photovoltaic Kohler concentrator is composed of a Fresnel lens 20 as the primary, and a single-surface imaging lens 21 as the secondary, which encapsulates the cell 22, as illustrated in FIG. 2. The primary images the sun onto secondary aperture 23. As the primary is uniformly illuminated by the sun, the irradiance distribution on the cell is also uniform, and it will remain unchanged when the sun moves within the acceptance angle (equivalently when the sun image 24 moves within the secondary aperture). The concentration-acceptance angle product that can be attained with this configuration is very limited, because the numerical aperture on the cell is small. Additionally, the system cannot be compact because the optic is refractive and uses a single Kohler integration element.